1. Field of the Invention
The invention relates to a method for the continuous production of woodbased and/or synthetic boards and to a continuously operating press for producing such boards.
2. Description of Related Art
One such method for producing wood-based and/or synthetic boards is described in DE A 20 58 820, in which a heating and a cooling section are used within a continuously operating press. The special feature of the apparatus used in conjunction with that method is the gas-permeable and/or liquid-permeable perforated steel belts. A disadvantage with this method, and its corresponding continuously operating press, is that there is a fixed association between the heating and cooling zones within the pressing section. The perforated steel belts leave unwanted structured surface markings on the finished board; and the pointwise supply of steam and gas through the perforated steel belts does not allow uniform processing over the entire width of the press and in the mat of pressing stock.
One reason that this method and its corresponding continuously operating press have not gained general acceptance is that the heat stored in the rolling supporting elements, which are in the form of rolling rods and chain carpets, would have to be removed in the cooling section before the cooling action could take effect on the product. Another reason is that the system operates in a relatively sluggish manner with respect to the cooling effect and the board to be produced. In addition, energy efficiency is relatively poor, which results from the fact that the cost of materials used to produce the energy has risen disproportionately.
In the conventional production of wood-based materials using resin binders, which may be phenol or formaldehyde based for example, the moisture in the form of water introduced into the particle or fiber board is used as a heat transfer medium to the core of the mat or board through the transition of this moisture into the vapor state of aggregation. The reaction starting temperature of the binder or glue at the center of the board is reached at an earlier or later point along the pressing section depending on the thickness of the board. This aforementioned vapor pressure, which is still effective in the finished board at the end of the continuous pressing operation, is in equilibrium or disequilibrium with the transverse tensile strength produced in the board by the curing process. If, for example, the production rate in the continuously operating press is too high, the board is destroyed as it leaves the continuously operating press as a result of vapor rupture which is caused by the disequilibrium of a board in which the glue has not yet completely set and the increased action of the vapor pressure.
One disadvantage of this continuously operating pressing method, which does not have an additional cooling zone, is that ruptures may result from excess vapor pressure acting in the hot board. A devaporization effect within such a continuously operating press may be achieved by utilizing a pressing-steam or pressing-pressure relief section. This additional processing section, however, is disadvantageous, because steam escapes as a result of the pressure drop introduced relative to the atmosphere outside the continuously operating press. For example, moisture (H.sub.2 O) in a range of from 2% to 3% is removed from the board, which has an unfavorable effect with respect to the swelling behavior and hence usefulness of a wood-based board, especially in climatic zones having high percentage atmospheric humidity.
Although the cooling action described in DE A 20 58 820 reduces the effective vapor pressure, allowing production with higher product reliability, it is associated with the economic disadvantage that there is only one fixed optimum operating point for production.
As disclosed in DE A 29 22 151, the use of separate arrangements for the heating and cooling zones has been preferred in industry. Each of these heating and cooling zones has independent circulation systems for the rolling support elements between the heating and cooling plates and the steel belts. One advantage of this arrangement is that each of these separate process areas uses energy quite efficiently. A disadvantage of such a continuously operating press, however, lies in the brief reduction in pressure towards zero N/mm.sup.2 in the transition area between the return of the hot rolling support elements at the end of the heating section and the inlet of the cold rolling support elements at the beginning of the cooling section. This can be critical, particularly in the case of slow transport speeds in the production of thick boards, because, in the case of wood-based boards, the vapor pressure in the core of the board bursts glued joints that have not yet set, i.e. the physical usage properties of the product are limited. Another serious disadvantage is the limitation to one optimum parameter setting, e.g. the production speed for a particular thickness of board.
This means, in general terms, that a system with fixed heating and cooling zones is uneconomical when used for a relatively wide range of widths in the production of thin (2.5 mm) boards to thick (38 mm) boards.
The difficulties suggested in the preceding are not intended to be exhaustive but are among many tending to reduce the desirability of known board production methods and known board pressing devices. Other noteworthy problems may exist; however, those presented above should be sufficient to demonstrate that those known methods and apparatuses are amenable to worthwhile improvements.